Population Equilibrium with Respect to Available Food Resource and its Behavioural Basis in an Herbivorous Lady Beetle, Henosepilachna niponica

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Abstract
(1) Population studies of a thistle-feeding lady beetle, Henosepilachna niponica (Lewis), were conducted from 1976 to 1980 at two different study sites to examine whether the local populations exist in a dynamic equilibrium state with respect to available food resources. Adult populations were estimated using the Jolly-Seber method based on mark-release-recapture data. Direct counts were applied to eggs and larvae on every thistle plant. (2) Food resource abundance measured as the number of thistle shoots changed from year to year rather independently in the two sites, but the egg populations surprisingly tracked the year-to-year changes in available food supply throughout the study period in each habitat. (3) The average egg density relative to food resource abundance over 5 years was almost identical between the two local populations. (4) After detectable external perturbations, mainly large-scale floods and arthropod predation, the egg populations quickly returned to the previous level of density. (5) It is concluded that the local populations of the lady beetle actually persist in a dynamic equilibrium, maintaining a constant density with respect to the amount of available food resource. (6) The egg populations approached, asymptotically, a plateau of a certain level of density per unit resource throughout the reproductive season in every year. (7) The possible regulatory process responsible for the population saturation involves the reduction in oviposition rate responding to cumulative egg load in the later season and, to a lesser extent, density-dependent losses of reproductive females probably due to dispersal in the early season. (8) The resorption of developing eggs in the ovary and frequent inter-plant movements of females for oviposition are causal behavioural bases producing a dynamic equilibrium state in the lady beetle and thistle system in every year.